The present invention is directed generally to point-of-sale (POS) printers and, more particularly, to ink compositions that are environmentally friendly, reliable, and fast drying for use in thermal ink jet printers for POS applications and check validation.
Thermal inkjet printers are well-known and have been extensively discussed elsewhere; see, e.g., Hewlett-Packard Journal, pp. 4-37 (May 1985); Hewlett-Packard Journal, pp. 6-56 (August 1988); Hewlett-Packard Journal, pp. 51-98 (October 1988); Hewlett-Packard Journal, pp. 64-102 (August 1992); and Hewlett-Packard Journal, pp. 6-97 (February 1994). Essentially, thermal ink jet printers use printheads that employ resistive heater elements to heat ink to boiling and thus propel the ink through an overlying orifice plate during an ink jet printing operation. Thermal ink jet printers are known (1) for their relatively fast throughput, similar to that of many laser jet printers, (2) for their relative inexpensiveness, lower than that of laser jet printers; and (3) for their ease of use and ease of print cartridge replacement.
Point-of-sale applications involve a variety of equipment, such as cash registers, credit card receipt printers, and the like. The typical print engine is a dot matrix impact printer or a direct thermal printer. Compared to ink jet printers, the former is well-known for its relatively noisiness, while the latter requires special paper.
Expansion of the utility of ink jet printers into point-of-sale (POS) applications is desired, due to the many advantages that ink jet offers over dot matrix impact printers and direct thermal printers, in particular, being much quieter than the former and not requiring special paper and lack of permanence upon exposure to heat and light, as is true of the latter. In addition, ink jet enables the use of color, which is neither available with dot matrix impact printers nor with direct thermal printers. However, there are several criteria that have to be met in order for thermal ink jet printers to find use in such applications.
First, the ink jet printer must be able to print receipts rapidly, thereby requiring high frequency printing, on the order of about 3 kHz (text). Second, due to the fact that the printer is used in public places, low toxicity is a requirement; ink compositions must be environmentally benign and non-toxic. Third, a print cartridge must be capable of printing on the order of several million characters before requiring changing, in order to minimize frequent cartridge changes. Fourth, the pen portion of the print cartridge is not likely to be capped during non-use, which means that the ink must be capable of long term decap, on the order of weeks or months. (Decap, as used herein, describes the ability of the pen to remain clear of plugs of dried ink in the nozzles.) Fifth, the ink must be fast drying. Previous inks were considered fast drying if they achieved drying times of 10 to 15 seconds; the drying time in such applications was dictated by the mechanics of the printer""s print speed and placing one printed page on top of another. In POS applications, however, the ink must dry nearly instantaneously, since upon completion of printing a receipt, it is immediately handed to the customer and therefore must not smudge the customer""s fingers. A drying time of less than 300 milliseconds (msec) is required to meet this condition, more than 10 times faster than previous inks were required to achieve. Sixth, since the ink jet printer is used repeatedly in POS applications involving the public, all parts must be able to handle minimum servicing. Seventh, the print quality, which is on the order of 100 dots per inch (dpi) must be no worse than other printing technologies, such as dot matrix impact or direct thermal. Eighth, and finally, the resulting print must exhibit adequate optical density (OD), by which is meant that the OD must be at least 0.3 for readability (on paper).
The foregoing requirements suggest that a proven, mature technology be employed, in order to minimize printer downtime. That is to say, it is better to use an older, existing product that can be modified than to develop a new product that might take years to prove out.
A printhead that meets many of the foregoing requirements is disclosed and claimed in U.S. Pat. Nos. 4,809,428; 4,500,895; and 4,509,062. Such a printhead employs a nickel orifice plate in combination with a glass (silica) substrate.
There are inks, for example, that are relatively fast drying, but are not designed for high frequency printing. Further, a problem that exists with the nickel orifice plate is that the inks developed for the foregoing printhead tend to puddle on the orifice plate, which interferes with printing crisp, clean images and lowers the overall system reliability.
Thus, there is a need for an ink jet ink that can meet the foregoing requirements for POS applications, while avoiding most, if not all, of the disadvantages of previously-developed inks.
In accordance with the teachings herein, an ink jet ink for thermal ink jet point-of-sale printers is provided, comprising:
(a) a vehicle comprising
(i) 5 to 10 wt % of a drying agent selected from the group consisting of 1,2-hexanediol and 1,2-pentanediol and mixtures thereof,
(ii) 15 to 32 wt % 2-pyrrolidone,
(iii) 15 to 37 wt % of at least one humectant, and
(iv) the balance water;
(b) 0.5 to 10 wt % of a dye or dye mixture; and
(c) 0 to 3% comprising at least one component selected from the group consisting of buffers, corrosion inhibitors, surfactants, biocides, polymers, pigments, and binders, with the proviso that the total amount of surfactants is 0.1 wt % or less, based on the total ink composition.
Further in accordance with the teachings herein, the ink jet ink is employed in combination with a printhead having a nickel orifice plate, through which droplets of the ink are expelled onto a print medium.
Additionally in accordance with the teachings herein, a method is provided for reducing puddling on the nickel orifice plate, comprising providing the foregoing composition and jetting it through orifice openings in the nickel orifice plate.
The ink jet ink composition is environmentally friendly, reliable, and fast drying, on the order of 300 msec and less. Further, the ink composition exhibits minimal puddling on the nickel orifice plate, leading to improved print quality.